Don't go chasing the mainstream - its not there any more. Whatever you are into thanks to the internet there is a forum or group or community about it - just find it and engage. Don't go in saying:
CHECK ME OUT I'M AMAZING
People will think - just another random spammer and ignore you. Have a genuine conversation, engage with the community and eventually people will discover your music naturally. As the very wise Steve Lawson said "Treat the internet like a country pub" - you wouldn't go into a bar and start shouting about how great you are would you? Of course not - they would think you are a nutter!!
Its far better to be honest and do the music you love, people can spot bullshit a mile off. Be passionate about what you do and talk to people about your music and make your music shareable so that others can spread the word.
Just be honest or no one will listen, people expect you to be real.
What Social Networks should I be on?
That depends to an extent what sort of music you play and your audience. The essentials are Twitter, Facebook, Myspace(even thou its rubbish, people still look there), your own email mailing list(use Reverbnation for this) and your own web site.
Ning networks are also useful if relevant to your niche - i use the these:
Go to Ning and do a search on Belgian Grindcore or Dutch Folk or whatever music you do and find your community.
Use the social media tools you are most comfortable with - video, podcasts, written word, photos or any combination you feel comfortable with. The more comfortable you are, the more honest you can be and the better connections you make.
You need a blog of your own - host it at blogger or wordpress, don't use the Myspace one its horrible! Write when you have something to say but try and keep it within once or twice a week. Post photos in it using Flickr then get the code from there to add photos to add to your blog.
Contact other bloggers do a Google Blog Search on a similar artist to yourself and start making friends with the bloggers that write about them. DO NOT SPAM!! Make friends - enjoy their blogs - comment, email to say you enjoy reading, engage and be honest. Then you may be in a position for them to write about your music. Another useful way to find bloggers is to hunt for them on hypem again using a similar artist.